There isn't really a fixed expression. It's generally understood that if you have reached the point of asking a salesperson to help you are at least in principle ready to buy.
However, if there are no other customers, and you really want to see what your feet look like in a particular pair of shoes you have options. You can either be honest:
Those shoes look amazing, could I try them on, even though I could never afford them.
Would it be possible for my just to try on those shoes. I can't buy them today
Alternatively, you can lie. Pretend that you have decided not to buy them
... Hmm, perhaps not, not really my style. Thanks anyway.
In the first case you can't complain if the sales person says "no". In the second the sales staff will quickly recognise you if you try it too often. It is rude to take up too much of their time, especially if there are genuine customers waiting.
I can't remember any shopkeeper offering to wrap anything in the UK (unlike in Japan) Perhaps I just don't go to the right type of shop. But if a salesperson had misunderstood me and asked "Do you want to pay", I'd just repeat what I had said earlier: "Sorry, no, like I said, they're beautiful but far to expensive for me", or (if I was pretending) "Sorry, they're not my style, I think I'll look elsewhere".